Palindromes in C++ - c++

Question

I'm trying to make a program that uses stacks (w pop, push, etc.) to read in a text file with lots of sentences that are taken one at a time and outputs whether each line is a palindrome or not (words that are spelled the same forwards and backwards). I believe its very close to being a completed program, but it only returns false even when the string is a palindrome. I want it to return true when the string is in fact a palindrome.
EDIT: Tried a new method with three stacks instead. Still getting a false return from bool tf every time.
int main() {
Stack s(100); // Initialize two different stacks
Stack q(100);
Stack temp(100);
string line; // String to hold each individual line of the file
char letter;
char x; // For comparisons
char y;
// open the file
ifstream input;
input.open(READFILE);
// Check that it is open/readable
if (input.fail()) {
cout << endl << "Sorry, file not available, exiting program. Press enter";
cout << endl;
cin.get(); // Grab the enter
return 0;
}
while (getline(input, line)) { // Read the file line-by-line into "line"
cout << "The line: " << line << endl;
int length = line.length(); // Sets length equal to string length
for (int i =0; i<length; i++){ // Capitalizes string
line[i] = toupper(line[i]);
}
for (int i = 0; i < length; i++) { // Loop through for every letter in the line
if (line[i] == ' ' ) {
line.erase(i,1); // Takes spaces out of the line
length--;
}
if (ispunct(line[i])){
length--;
}
if (!ispunct(line[i])){ // Removes punctuation
letter = line[i]; // Push each letter onto the stack
s.push(letter);
}
}
for (int i = 0; i < length; i++) { // Popping half the letters off of the s stack
s.pop(letter); // and pushing them onto the q stack
q.push(letter);
temp.push(letter);
}
for (int i = 0; i < length; i++) {
temp.pop(letter);
s.push(letter);
}
bool tf = true; // Pop off the top of each stack and compare
while (!s.empty()) { // them to check for a palindrome
s.pop(x);
q.pop(y);
if (x == y);
else tf = false;
}
if (tf){
cout << "is a palindrome!" << endl;
}
if (!tf) {
cout << "is NOT a palindrome" << endl;
}
}
}

Answer 1

for (int i = 0; i < length/2; i++) // Popping half the letters off
//of the s stack
q.push(letter); // and pushing them onto the q
//stack
}
Here you're pushing the same letter over and over again.
Even if you rewrite as the comment states it will be wrong.
if you pop half of ABBA you have BA and AB and compare B=A
you need to rethink your strategy. Maybe push half of the string to s then loop backwards from length and push to q

Answer 2

Like someone else mentioned, even after fixing the for loop with the "q" stack, the general strategy is not correct. In fact you don't need two stacks. (or even one stack, but you can use a stack if desired.)
You do have the right idea in comparing the back half of the letters with the front half. In general, to find a palindrome you just need to see if the string is equal to the reversed string or that the first half is equal to the back half.
You can use a stack to store the string in reverse. All you need is the stack and the string. However, there is the extra problem here in that the lines of strings contain spaces and punctuation that you want to ignore. Using the erase() method reduces the length of the string as you go, so you need a temporary variable to rebuild the formatted string at the same time as the stack. EDIT: I saw your update to accounting for the reduced length; that's great -- it can save even the use of a temp variable to hold the formatted string so that the variable string line is all that is needed.
Here is another version of your while loop that uses one stack and a temp string variable. It uses half the formatted string to compare against the top of the stack (which represents the "back" of the string).
string cleanString;
//cout << "test3";
while (getline(input, line)) { // Read the file line-
//by-line into "line"
cout << "The line read was: " << line << endl;
int length = line.length(); // Sets length equal to
//string length
for (int i =0; i<length; i++) // Capitalizes string
line[i] = toupper(line[i]);
for (int i = 0; i < length; i++) // Loop through for //every letter in the line
if ( !(line[i] == ' ' || ispunct(line[i]))) { // Ignore space & punctuation
letter = line[i]; // Push each letter onto
s.push(letter); //the stack
cleanString.push_back(letter); //and to the "cleaned" string to compare with later
//cout << cleanString << endl; //test
}
length = cleanString.length();
bool tf = true;
for (int i = 0; i < length/2; i++) { // Pop off the top of stack
s.pop(x); // to compare with front of string
if ( cleanString[i] != x ) { //not a palindrome
tf = false;
break;
}
}
if (tf){
cout << "is a palindrome!" << endl;
}
if (!tf) {
cout << "is NOT a palindrome" << endl;
}
}
But it's simpler to skip the use of the stack altogether and instead just use the temp "cleaned" string, checking for a palindrome in a for loop with two counters: one for the the front and one for the back.
So after the capitalization:
// Instead of a stack, just build a string of chars to check for a palindrome
for (int i = 0; i < length; i++)
if ( !(line[i] == ' ' || ispunct(line[i]))) {
letter = line[i]; // Push each letter onto
cleanString.push_back(letter); // a temp string
}
length = cleanString.length(); //use length of formatted string
bool tf = true;
int front = 0; // first char of string
int back = length-1; // last char of string
for (; i < length/2; front++, back--)
if ( cleanString[front] != cleanString[back] ) { //not a palindrome
tf = false;
break;
}
Another option is to use the inbuilt reverse() function in the <algorithm> header file after building the temp string:
#include <algorithm> // reverse()
string cleanString;
string reversedCleanString;
//...
// Instead of a stack, just build a string of chars to check for a palindrome
for (int i = 0; i < length; i++)
if ( !(line[i] == ' ' || ispunct(line[i])))
cleanString.push_back(line[i]);
reversedCleanString = cleanString; // store copy of string to reverse
reverse(reversedCleanString.begin(), reversedCleanString.end() ); // reverse
bool tf = true;
if ( cleanString != reversedCleanString)
tf = false;
// ...
As moooeeep mentioned the comments, using std::string's reverse iterators simplifies this even further after the capitalization:
string cleanString;
//...
// Format line to test if palindrome
for (int i = 0; i < length; i++)
if ( !(line[i] == ' ' || ispunct(line[i])))
cleanString.push_back( line[i] );
bool tf = true;
if ( cleanString != string(cleanString.rbegin(), cleanString.rend() )
tf = false;
// ...
Also, like moooeeeep mentioned, encapsulating the different parts of the while loop into their own separate functions is a good idea to make not just debugging easier but also understanding the logical flow of the problem more intuitively.
For example the while loop could look like this:
while (getline(input, line)) { // Read the file line-by-line into "line"
//echo input
cout << "The line read was: " << line << endl;
// validate/format the line
extractChars( line ); //remove space/punctuation
capitalizeString( line ); // capitalize chars for uniformity
//check if formatted line is a palindrome and output result
if ( is_palindrome( line ) )
cout << "Line IS a palindrome << endl;
else
cout << "Line IS NOT a palindrome << endl;
}